Pharm-Immunology-14 & 15

Immunology of Cancer & Transplants

Hussein

Introduction

• Immunological surveillance is proposed as means of detection of transformed cells by the immune system.

• Immune reactions could be induced at later stage and contribute to limiting of tumor development.

• Third, therapeutic use of the immune system is more than a possibility

Objectives

1. Know the basis of immune recognition of tumors

2. Know the immune responses to tumors

3. Know the concept of immunosurveillance

4. Know the mechanisms of immunological escape

5. Understand the potential of immunological cancer therapy

6. Understand the principles of immunological cancer diagnosis

Evidence supporting the concept that the immune system reacts against tumors

• Several lines of clinical and experimental evidence indicate that defense against tumors is mediated by reactions of the adaptive immune system

Fig. 10-1

Immune Recognition• Tumor cells undergo some changes that include some

surface antigens enabling the immune system to recognize them as different from normal cells. Examples:

1. Cancers associated with viral infections might express viral encoded antigens

2. Normally silent genes are expressed in tumor cellsOncofetal antigens:a) a-fetoprotein (AFP) in hepatic cancerb) carcinoembryonic Ag (CEA) in intestinal cancers

3. Ags resulting from mutations (point mutations; e.g. p53)4. Abnormal surface carbohydrates with new antigenic activity

5. Changes on the surface of cycling cells1. Cancer cells are faster dividing than normal cells2. Important: 95 kDa glycoprotein, F19 in sarcomas & several carcinoma

6. CALLA is an antigen associated with childhood leukemia

CALLA (CD10)1. Common Acute

Lymphoblastic Leukemia Antigen normally expressed only on B-cell progenitors (lymphoblast)

• They make up < 1% of normal bone marrow cells

2. CALLA becomes more abundant in the commonest childhood leukemia

Immune Responses

• T Lymphocytes:– CD8+ CTLs rejection of transplanted tumors

• MHC I peptide presentation is necessary for recognition & killing

• Tumor-infiltrating lymphocytes (TILs) contain CTLs capable of lysing the tumor of origin

• CD4+ helper T cells provide:– cytokines, including:

i) tumor necrosis factor (TNF),

ii) interferon- (IFN-)

Specificity of tumor immunity

• Production of a highly immunogenic (tum-) variant tumor in DBA2 mice and Tc cells specific for it:

– Inducing mutation by irradiating of the parent cells

– Subclone tumor cells:

• Some subclones (tum-)

are no longer able to grow in DBA2 mice

• Spleen Tc from mice injected with these tum- cells could kill tum- but not the parent tumor

tum-

Tc specificity • Tc cells taken from a mouse immunized with a tum- variant tumor: (in vitro)– kill tumor cells

coated with a peptide from the tum- gene sequence

– They do not kill tumor cells coated with the homologous peptide from the parental tumor

• The two peptides differ in one amino acid

Immune ResponsesNatural Killer Cells:

i. Kill by lysis• no expression of T cell antigen receptors• MHC independent killing

ii. Kill viral infected cells & hematopoietic tumors in vitro

iii. Express low-affinity Fc-receptors for IgGiv. Cytokines (IFNs, TNF, IL-2, IL-12) increase the

tumoricidal capacity of NK cellsv. IL-2 activated NK cells are called lymphokine-

activated killer (LAK) cellsvi. Role in immunosurveillance: Nude mice w/o high

incidence of spontaneous tumors

Immune Responses

• Macrophages mechanisms of killing:

1. release of lysosomal enzymes

2. reactive O2 metabolites

3. nitric oxide in mice

4. TNF secretion. By what mechanisms does TNF kill tumor cells?

• TNF [also called cachectin] is a polypeptide cytokine, produced by endotoxin-activated macrophages, which

1. has the ability to modulate adipocyte metabolism,

2. lyse tumor cells in vitro, and

3. induce hemorrhagic necrosis of certain transplantable tumors in vivo

Immune Responses

Antibodies:

• There is some evidence that cancer patients produce antibodies against their own tumors

• Humoral immune response is far less important than the cellular in anticancer defense

Immunosurveillance

• It is performed by the immune system using the same mechanisms discussed above to discover any developing cancer and destroy it

• NK cells play an important role in surveillance

• It does not work all the time– Proof: there are cancer patients

Evasion Of The Immune System/Escape mechanisms1- Down-regulation of MHC I on tumor cells2- Lack of MHC II in human tumors no direct activation of CD4+

helper T cells3- Lack of costimulators (B7) may impair T cell activation4- Suppression of anti-tumor immune responses by tumor products5- Host tolerance to tumor antigens due to

a) neonatal exposure or b) presenting antigens in tolerigenic form

6- Selection of mutant tumor cells lacking immunogenic peptide-MHC complexes.

7- Antigenic modulation due to endocytosis or shedding of the antigen-antibody complexes

8- "Sneaking through" phenomenon based on the kinetics of tumor growth that allow establishment of resistant tumors.

9- Hiding of tumor cell surface antigens by glycocalyx molecules and others such as coagulation induction fibrin cocoon.

How tumors evade immune responses?

• Antitumor immunity develops when T cells recognize tumor antigens and are activated

• Tumor cells may evade immune responses by:– losing expression of antigens

– loosing MHC molecules

– producing immunosuppressive cytokines

Fig 10-4

Tumor Ag presentation to T cells in 3 ways:1. Directly in the absence of necessary co-stimuli, resulting in anergy.2. Directly by a tumor which expresses co-stimulatory molecules,

resulting in Tc cell activation. 3. Directly by tumor cells and indirectly via specialized APCs, resulting in

activation of both Tc and TH cells

TCR

Cancer Immunotherapy1- Active immunization by using oncogenic viral proteins as

vaccines against this class of tumors.2- Protection against some tumors can be achieved by immunization

with chronic lymphocytic leukemia (CLL) cells idiotype fused with GM-CSF

3- Transfection of weakly immunogenic tumors with costimulatory molecules (B7) and cytokines (IFN, IL-2, IL-4, IL-7)

4- Monoclonal antibodies as magic bullets when conjugated with toxins or radionuclides to target tumor cells or antigens on malignant fibroblasts.

5- Harnessing of innate immune mechanisms:i. LAK against renal carcinoma;ii.IFN- and a very effective against T-cell disorders (hairy cell leukemia & mycosis fungoides);

less effective against Kaposi's sarcoma & various lymphomasiii.Combining therapies with synergistic effects.

Strategies for enhancing antitumor

immune responses

Tumor-specific immune responses may be stimulated by:

A. Vaccinating with host dendritic cells that have been pulsed with tumor antigens

B. Vaccinating with plasmids containing cDNA encoding tumor Ags that are injected directly into patients or used to transfect dendritic cells

C. Vaccinating with tumor cells transfected with genes encoding B7 costimulators or the T cell growth factor IL-2

Fig 10-5

Pulsing of dendritic cells

Antihelminthic, immune Response

enhancer

Augmenting the host response by specific

immunotherapy

Therapeutic modification of mAbs1. Genetically engineered

chimeric Abs with human Fc portion attached to mouse Fab 2 1. reduce the risk of an

immune response to the mAb

2. Human Fc will also recruit human effector mechanisms

2. Molecules that can be coupled to mAbs1. Toxins: Ricin2. Cytotoxic drugs or

enzymes capable of activating drugs

3. Radioactive isotopes

In vitro purging of tumor-infiltrated bone marrow

• Bone marrow containing

tumor cell can be purged

using:

1. mAbs and complement,

2. Ab-toxin conjugates or

3. Ab coupled to magnetic

beads

• Store purged marrow

• Treat patient with

radiation and chemotherapy

• Treat purged marrow with anti-tumor Ab

• Return bone marrow to patient

• Therapy results are encouraging in leukemia and lymphoma patients who where not helped by conventional therapy

Immunodiagnosis1- Circulating tumor markers are diagnostic:

i. -fetoprotein (AFP) hepatic carcinomaii. Carcinoembryonic Ag (CEA) colorectal carcinomaiii. GM1 monosialoganglioside in 96% of patients with pancreatic carcinoma

2- Monoclonal Ab to tumor surface antigens can provide a basis for imaging. Good targets include:

i. F19 glycoprotein on reactive stromal fibroblastsii. Certain tumor mucins/epithelial cancers (T antigen)iii. Cytokeratin on carcinoma cells

3- Detection of bone marrow micrometastasis using immunocytochemistry techniques provides information on:i. Prognosisii. Efficacy of a new therapyiii. Eventual recurrence of treated cancer

NORMAL RANGE

Immune Response Against Transplant• Know the evidence for the immune nature of transplant

rejections• Describe MHC Ags as the principal targets of rejection• Know the the types of graft rejection:

– Acute– Hyperacute– Chronic

• Understand the role of immunosuppressive drugs such as cyclosporine in organ transplantation

• Describe the principle of mixed lymphocyte reaction (MLR)

• Know blood transfusion• Know the bone marrow transplantation and the associated

graft-versus-host (GvH) disease

Types of grafts• Syngeneic grafts (syngraft)

– Donor and recipient are genetically identical No rejection• Identical twins• Inbred mice• Their Ags are called isoantigens• Isoantibodies

• Allogeneic grafts (allograft)– Donor and recipient are genetically different members of the

same species Rejection• Their Ags are called alloantigens• Their Abs & T cells are alloreactive (alloantibodies)

• Xenogeneic (xenograft)– Donor and recipient are from different species Rejection

• Their Ags are called xenoantigens• Their Abs & T cells are xenoreactive

Types of Grafts

1. Autograft: Autogenic, autologous tissue

2. A. Syngraft: Syngeneic tissue

B. Isograft: Isologous tissue

3. Allograft (Homograft): Allogeneic tissue

4. Xenograft (Heterograft): Xenogeneic (Heterologous) tissue

Transplantation Laws

1. Syngrafts are accepted

2. Allografts are rejected

3. Transplants from parent to F1 hybrid are accepted; reverse rejected

4. Transplants from F2 individuals to F1 are accepted*

5. Transplants from either parent are mostly rejected, but accepted in some

Evidence indicating that the rejection of tissue transplants is an immune reaction

• Clinical and experimental evidence indicates that rejection of grafts is a reaction of the adaptive immune system

Recognition of allogeneic MHC molecules by T

lymphocytes.

• Recognition of allogeneic MHC molecules may be thought of as a cross reaction in which a T cell specific for a self MHC molecule-foreign peptide complex (A) also recognizes an allogeneic MHC molecule whose structure resembles that of a self MHC molecule-foreign peptide complex (B, C)

• Donor peptides derived from the graft may not contribute to allorecognition (B), or

• They may form part of the complex that the T cell sees (C)

• The type of T cell recognition depicted in B and C is called direct allorecognition

Direct and indirect recognition of alloantigens• A. Direct alloantigen recognition occurs when T cells bind directly to

intact allogeneic MHC molecules on professional APCs in a graft

• B. Indirect alloantigen recognition occurs when allogeneic MHC molecules from graft cells are taken up & processed by recipient APC

and peptide fragments of the allogeneic MHC molecules are presented by recipient (self)

MHC molecules

• Recipient APCs may

also process &

present graft proteins

other than allogeneic

MHC molecule

GraftAPC

Mechanisms of graft rejection:Hyperacute rejections

• In hyperacute rejection, preformed antibodies react with – alloantigens on the vascular endothelium of the graft,

– activate complement, and

– trigger rapid intravascular thrombosis and necrosis of the vessel wall

Mechanisms of graft rejection: Acute rejection

• In acute cellular rejection, CD8+ T lymphocytes reactive with alloantigens on graft endothelial cells and parenchymal cells cause damage to these cell types

• Endothelium inflammation is sometimes called "endothelialitis" • Alloreactive antibodies may also contribute to vascular injury

Chronic Graft Rejection

1. Months or years after Transplant

2. Example:

– Kidneys gradual loss of function

3. Biopsy reveals rejection process while progressing slowly

Mechanisms of graft rejection: Chronic rejection

• In chronic rejection with graft arteriosclerosis, T cells reactive with graft alloAgs produce cytokines that induce endothelial & intimal smooth muscle cells proliferation, leading to luminal occlusion

• This is probably a chronic DTH reaction to alloantigens in the vessel wall

Graft Rejection Displays Immunological Memory

• Left:– A human skin allograft @ day 5 is fully vascularized & cells are

dividing• Middle:

– Graft is totally destroyed by day 12• Right:

– Second-set graft from the same donor fails to vascularize and is destroyed rapidly by day 7 because the first graft sensitized the recipient Immunological memory

Graft-versus-Host Reaction (GvHR)

1. The graft rejects the host2. Graft cells are immunocompetent3. Recipient is immunoincompetent4. Conditions for GvHR:

i. Histoincompatibilityii. Transfer of immunocompetent (T) cellsiii. Immunodeficient recipient

5. GvHR occurs after bone marrow transplantation for stem cell replacement in:i. Aplastic anemiaii. Acute leukemiaiii. Combined immunodeficiencies

Mixed lymphocyte reaction

• MLR is an in vitro test for T cell recognition of alloantigens

• T cells from the potential recipient of a transplant are cultured with WBCs from the potential donor

• The response of the T cells is assayed– If the response with lysis and cell damage is strong

the matching between the two individuals is weak– The weaker the response, the greater the match

• This is a rough predictor of the transplant’s outcome: rejection or acceptance

ABO blood groups & blood transfusion• Blood group Ags are a major barrier for blood transfusion• ABO Ags are glycosphingolipids mainly on surface of RBC

– Group A has N-acetylgalactoseamine Ag & anti-B Ab– Group B has galactose Ag & anti-A Ab– Group AB has both Ags & no Abs– Group O has neither Ag & both Abs (anti-A & anti-B)– Transfusion reaction occurs when donor has Ags that are not

present in the recipient

• The Abs are sometimes called natural Abs and thought to be made against intestinal bacteria and capable of cross-reaction with the blood group Ags

• Rh groups are important especially in cases of pregnancy where the mother is Rh- and the baby is Rh+

– In the second pregnancy the baby will have problems of blood lysis

Treatments for graft rejection

• Agents that are commonly used to treat the rejection of organ grafts, and the mechanisms of action of these agents, are listed

• FK506 is a drug that works like cyclosporine, but FK506 is not used as widely

(Tacrolimus)

Brendan Doran